1. Field of the Invention
The present invention concerns a positive active material for nickel electrodes for alkaline electrolyte storage batteries. It also extends to the method of preparing the active material and to its use in the manufacture of the electrode
2. Description of the Prior Art
Most recent applications of alkaline electrolyte storage batteries oblige the user to charge the batteries at temperatures higher than room temperature. This is the case when the storage batteries are used in vehicle traction, for example. When the charging temperature is in the order of 40xc2x0 C. to 60xc2x0 C., the efficiency of charging the positive electrode is poor because the oxygen release excess voltage at the end of charge is low.
U.S. Pat. No. 5,466,543 describes a paste type electrode for alkaline storage batteries adapted to operate over a wide range of temperatures. The paste contains an active material and at least one compound of yttrium, indium, antimony, barium, beryllium and at least one compound of cobalt or calcium.
European patent application 0 634 804 proposes a paste comprising a mixture of active materials consisting mainly of nickel hydroxide and containing at least one element selected from cobalt, cobalt hydroxide, cobalt oxide, carbon and nickel and a powder compound of at least one element chosen from Ca, Sr, Ba, Cu, Ag and Y.
The efficiency obtained with such electrodes is still insufficient for some applications. These poor results are associated with unsuitable distribution of the substances in the paste and the formation of lumps which renders some of the additives ineffective.
The present invention consists in a positive active material for nickel electrodes for alkaline storage batteries consisting of particles of hydroxide containing mainly nickel and covered with a layer of a hydroxide phase based on nickel and yttrium, the proportion of the hydroxide phase being in the range 0.15% to 3% by weight of yttrium expressed as yttrium hydroxide relative to the total weight of particles.
Because the hydroxide particles are coated with the layer of the hydroxide phase, the latter is distributed in a highly homogeneous manner throughout the active material, which cannot be achieved with the mixtures of powders of the prior art. Moreover, the phase can exert its influence on each hydroxide particle, with maximum efficiency.
It is therefore possible to minimize the quantity of the hydroxide phase to be used, enabling the electrode to retain a high level of filling with electrochemically active material and therefore a high energy per unit volume.
Accordingly, from 0.15% by weight of yttrium relative to the total weight of particles, the efficiency of the active material is significantly increased, in particular when charging at high temperature.
Beyond 3% by weight, this improvement is canceled out by the reduced proportion of electrochemically active material, the yttrium contained in the hydroxide phase not contributing to the electrochemical reactions.
The hydroxide phase is preferably of the hydrotalcite type with indexed lattice parameters in the hexagonal system a=3.122 xc3x85xc2x10.01 xc3x85 and c=7.567 xc3x85xc2x10.01 xc3x85.
The hydroxide phase can further contain at least one element such as cobalt co-crystallized with the nickel and the yttrium.
The hydroxide phase containing nickel and yttrium preferably also occupies at least part of the surface microporosity of the hydroxide particle. This is the microporosity accessible to the electrolyte and which contributes to the electrochemically active surface area of the hydroxide.
The hydroxide particles can be of any shape, from a more or less spherical shape to a totally irregular shape.
The hydroxide of the particles advantageously further contains in solid solution at least one hydroxide selected from the hydroxides of cobalt, aluminum, iron, copper, chromium and manganese and at least one hydroxide selected from the hydroxides of cadmium, zinc and magnesium.
In this case the hydroxide particles still contain mainly nickel and usually a minor proportion, not exceeding 20%, of hydroxide of one or more of the other co-crystallized elements. The crystallized structure of the hydroxide is not modified.
The present invention also consists in a method of manufacturing the above active material comprising the following steps:
a solution is prepared containing yttrium ions and nitrate ions,
the hydroxide particles are immersed in the solution,
the particles are separated from the solution and the particles are drained,
the hydroxide particles are immersed in an alkaline solution,
the particles are separated from the alkaline solution, and
the particles are washed with water and dried.
Of course, by xe2x80x9chydroxide particlesxe2x80x9d we mean particles in which the hydroxide contains mainly nickel but also the previously mentioned co-crystallized hydroxides.
The solution containing yttrium ions is preferably a solution of yttrium nitrate, but can be selected from a mixture of a solution of potassium nitrate, sodium nitrate or lithium nitrate and a solution of yttrium sulfate, yttrium chloride or yttrium acetate.
The alkaline solution is preferably selected from sodium hydroxide NaOH, potassium hydroxide KOH and lithium hydroxide LiOH.
A time of contact with the alkaline solution in the range 30 minutes to five hours is sufficient, but the immersion time can be as much as 48 hours without disadvantage.
The drying temperature is in the range 40xc2x0 C. to 110xc2x0 C. for a time in the range 12 hours to 48 hours.
The present invention also consists in an alkaline storage battery positive electrode using the above active material; an electrode of this kind comprises a conductive support and a paste containing a binder, a conductive powder and the active material of the invention.
The support is a porous three-dimensional conductive support such as a nickel sponge.
The binder includes at least one compound selected from carboxymethylcellulose CMC, hydroxyethylcellulose HEC, hydroxypropylmethylcellulose HPMC and at least one compound chosen from polytetrafluoroethylene PTFE and polyvinylidene fluoride PVDF.
The conductive powder is chosen from metallic cobalt Co, cobalt hydroxide Co(OH)2, cobalt oxide CoO and mixtures thereof.
An electrode in accordance with the invention can be used in any alkaline storage battery and in particular in storage batteries having cadmium or hydridable alloy negative electrodes.
Other features and advantages of the present invention will become apparent from the following examples given by way of illustrative and non-limiting example and from the appended drawing.